Campaigning in New Hampshire, Perry accused the former Massachusetts governor of changing positions on gun control, the causes of global warming and government health insurance mandates.

He is seeking to chip away at Romney's big lead in the polls in a key early primary state, and arrest his own slide in national surveys.

"Like it or not the governor has been on the opposite side of a lot of issues," Perry said during a live interview with conservative activist and New Hampshire Republican gubernatorial candidate Ovide Lamontagne. "The issue is who are you really going to trust to stand up and be consistent?"

The rivalry between the two candidates seems likely to intensify. Both candidates scheduled events in Manchester, New Hampshire, on Friday evening.

Both Romney and Perry focused their criticism on each other rather than businessman Herman Cain who now leads in some national Republican polls, an indication that neither campaign views Cain as a serious threat.

Also piling on Romney on Friday was former Utah Governor Jon Huntsman, who is preparing to make a four-day campaign swing through New Hampshire.

"Real leadership is taking a clear position on issues even if it comes at political risk. Backflipping is for toys and gymnasts, not presidents," the Huntsman campaign said in a new web video.

Perry, who briefly led in national polls after entering the Republican race in August, has faded after a string of shaky performances in candidate debates.

In Concord on Friday he sought deflect criticism from Romney and Tea Party conservative activists who accuse him of being soft on illegal immigration.

Perry signed a bill that allowed Texas residents without legal U.S. resident status to attend Texas colleges, while paying in-state tuition. The bill passed the Texas legislature with overwhelming bipartisan support.

"We could kick these people to the side of the road and then we'll have to pick up the costs of whatever those social programs they're going to be eligible for," he said. "We're going to either have tax wasters or tax payers."

Perry's campaign also chided Romney for allowing people without proof of citizenship to access health care programs for the poor while governor of Massachusetts, an indication of how strongly the illegal immigration issue is resonating with Republican primary voters.

The Texas governor, who has indicated he may skip some of the remaining Republican debates, joked about his debating shortfalls and sought to minimize the importance of debate performances.

"Shoot, I may be a great debater before it's all over," he said. "We have a very very good debater and a slick politician in the White House right now, and it's not working."